1. Field of the Invention
The subject invention relates generally to exhaust systems, and more specifically to the design and location of reinforcing structures on an exhaust system component for minimizing vibration related noise.
2. Description of the Related Art
The exhaust gas system of an automotive vehicle channelizes exhaust gas from the engine to a location where the exhaust gas can be emitted safely. The exhaust system also attenuates noise associated with the engine combustion and the flowing exhaust gas. A typical exhaust gas system includes at least one exhaust pipe that extends from the engine, at least one exhaust muffler that communicates with the exhaust pipe and at least one tail pipe that extends from the muffler. A catalytic converter generally communicates with the exhaust pipe between the muffler and the engine.
The prior art exhaust muffler includes an inlet that communicates with the exhaust pipe, an outlet that communicates with the tail pipe and a plurality of internal tubes and chambers that permit a controlled expansion of the flowing exhaust gas and creates acoustic altering components. The expansion of the exhaust gas dissipates the energy associated with the flowing exhaust gas and significantly reduces noise levels. Noise levels are reduced when they encounter acoustic altering components.
Engineers can design the internal components of a muffler based on exhaust gas flow characteristics and acoustic output of the engine. The design process generally is iterative. Thus, a prototype muffler may be developed based on flow characteristics and acoustic output of the exhaust gas. The prototype muffler then is bench tested with the engine, and noise output is analyzed. The array of tubes and chambers in the muffler then may be altered in an effort to optimize the performance of the muffler.
Most prior art mufflers comprise an array of conventional cylindrical pipes that are supported parallel to one another by a plurality of transverse baffles. The subassembly of pipes and baffles is slid into a tubular outer shell so that the baffles and the outer shell define chambers within the muffler. Some tubes are perforated within certain of the chambers, while other tubes may dead end within a chamber. Opposed end caps or headers are mounted to opposite ends of the tubular outer shell. One end cap typically is provided with an inlet to which the exhaust pipe is mounted. The opposed end cap typically is provided with an outlet to which the tail pipe is mounted.
The prior art also includes stamp formed mufflers. A stamp formed muffler includes plates that are stamped to define channels. The plates are secured in opposed relationship to one another so that the channels register. A registered pair of channels defines the functional equivalent of a conventional tube. The prior art stamp formed muffler further includes a pair of stamp formed outer shells that are secured around the tubes defined by the internal plates. Peripheral portions of the outer shell and at least one of the internal plates are secured to one another to define the chambers that communicate with the tubes formed by the internal plates. The outer shells further are formed to define at least one inlet and at least one outlet.
Exhaust system components must compete with other required components of a vehicle for the limited available space on a vehicle. Conventional tubular mufflers have few options for the size, shape and location of inlets and outlets. Thus, conventional tubular mufflers are not well suited for the many applications where the available space is very limited. Stamp formed mufflers, on the other hand, are not limited to a tubular shape and do not require the inlet and outlet to be on opposite ends of the muffler. Hence, stamp formed mufflers provide more design options than conventional tubular mufflers and are more desirable in many situations.
The noise associated with an automotive exhaust system is not limited to noise generated by the flowing exhaust gas. More particularly, forces exerted by the flowing exhaust gases and forces created by the acoustic and vibration energy of the engine cause panels of both a conventional tubular muffler and a stamp formed muffler to vibrate. The vibrations that coincide with the natural frequencies in the shell of the muffler are amplified. The first several natural frequency modes can generate objectionable noise independent of the noise associated with the exhaust gas.
Exhaust system manufacturers typically have dealt with the problem of vibration related noise by forming ribs in the outer shell and by providing a separate outer wrapper. The ribs and the outer wrapper are intended to provide enhanced rigidity, and to thereby minimize vibration related noise. The design and location of ribs generally has not been very scientific. A typical muffler with a tubular outer shell will include an array of parallel spaced apart ribs that extend longitudinally along the muffler. The spacing and size of the ribs on conventional tubular mufflers has been dictated mostly by the equipment used to create the ribs, and hence has not varied significantly from one muffler to another. Some muffler manufacturers consider their rib pattern to function as a trademark, and hence there has been little incentive to optimize the rib design. Stamp formed mufflers also have included parallel ribs. Although stamp formed mufflers have taken many shapes, the ribs typically have extended generally transverse to the longitudinal direction of the muffler. Slight variations in the rib pattern on a stamp formed muffler might be made as part of the above-described iterative design of a muffler. However, such design variations typically would follow the prevailing trend of parallel ribs, and redesign efforts typically have been based on trial and error.
Exhaust system manufacturers are under substantial pressure to reduce the weight of an exhaust system. Additionally, automobile manufacturers typically out-source the design and manufacture of exhaust systems, and price is an important factor in the selection of a supplier. Cost and weight savings can be achieved by employing thinner metal for the muffler or by eliminating the outer shell. However, vibration related noise is likely to increase when thinner metal is used for the muffler or when an outer shell is eliminated.
Software has been developed by Altair Engineering and sold under the trademark OPTISTRUCT® to identify locations on panels of a muffler, oil pan or the like that will vibrate at selected natural frequencies. The software is employed by inputting data to define the size and shape of the panel. The software then identifies locations that will vibrate at selected natural frequencies and outputs a theoretical shell geometry that would substantially reduce vibrations at the selected natural frequencies. The theoretical shell geometry, however, generally will require a three-dimensional matrix with tens of thousands of intersecting surfaces. Hence, the theoretical shell geometry produced by the OPTISTRUCT® software is acknowledged to be unmanufacturable, and merely is used as a guide for developing a more effective pattern of parallel ribs. For example the OPTISTRUCT® identification of locations that will vibrate at the selected natural frequencies and the theoretical shell geometry may be presented to an engineer who will design parallel ribs at locations that will vibrate at the selected natural frequencies and at locations that appear to require reinforcement for other reasons. The geometric changes that result from this proposed rib pattern will be inputted to the OPTISTRUCT® software, and a new simulation will be run to determine whether vibrations at the selected natural frequencies have been avoided. Alternatively, the engineer may input data regarding minimum rib width, recommended cross-sectional angles for each rib and maximum rib depth. The software then will recommend one or more optional rib patterns that will eliminate or substantially reduce vibration at the selected natural frequencies. Thus, the OPTISTRUCT® software can be used as part of an effort to reduce weight and costs.
An object of the invention is to provide an efficient method for designing ribs in a muffler to provide optimum resistance to vibration related noise with reduced material thicknesses.